Anaerobic digestion of organic waste has been implemented for many years. In anaerobic digestion, a mixed culture of bacteria mediates the degradation of the putrescible fraction of organic matter ultimately to methane, carbon dioxide and mineralized nutrients. Upon storage, organic waste will naturally begin this process of degradation, resulting in the production of intermediate compounds, which are volatile and often a source of odors. Since methanogenic microorganisms grow slowly and are present in limited numbers in fresh manure, these volatile intermediates accumulate in stored manure. In an effective anaerobic digester, the growth of methanogens is promoted such that the intermediate compounds are converted to biogas and nutrients, and the odor potential of the waste is greatly reduced.
The principal means for promoting methanogenic growth in anaerobic digestion of organic waste are controlling the operating temperature and/or controlling the residence time of the bacteria within the process. The types of anaerobic digester that have been implemented in the digestion of organic waste, in particular agricultural waste such as manure, are rather limited due to the nature of the waste, in particular manure, as a substrate.
Types of anaerobic digesters available for bioprocessing of organic wastes include plug-flow digesters, complete-mix digesters, covered lagoons and a few demonstrations of mixed reactors with flexible-film support media (U.S. Pat. Nos. 5,096,579; 6,254,775), all of which are variations of batch and semi-continuous processes. Except for covered lagoons, these anaerobic digesters are usually operated at mesophilic temperatures (usually 35° C.), which requires energy input. Often a portion of the biogas is used to heat the waste slurry to the operating temperature. The requirement for heating dictates that the waste slurry fed to these reactors should have as high a total solids (TS) content as possible to minimize the water content which must be heated. In practice, the waste slurry added to these heated digesters should have a TS content of 4-12%. In temperate climates, often waste slurry with 1% TS or less will fail to provide enough biogas to heat the slurry to 35° C.
Without any support media for bacterial residence/attachment, plug-flow and complete-mix organic waste digesters rely on the hydraulic retention time (HRT) to control the solids residence time sufficiently to promote methanogenic growth. At mesophilic temperatures, effective treatment dictates that the HRT be maintained at greater than 10 days and, in practice, a 20-40 day HRT is common. The volume of the digester is directly proportional to the chosen HRT and the volumetric rate of waste production. Again, like temperature, the long HRT requirements of these anaerobic digesters dictate that feed waste slurries must have as high a TS content as possible to minimize excess water, which takes up digester volume and results in a higher digester volume requirement to achieve the design HRT.
Currently, when treating manure waste, many livestock facilities use large volumes of water for barn flushing, resulting in excessive amounts of dilute wastewater (<1% TS). This effectively precludes these operations from using conventional plug-flow and complete-mix manure digesters due to both the uneconomical digester volume requirements and the excessive energy required to heat the dilute manure to mesophilic temperatures for effective digestion. Ideally, an anaerobic digestion apparatus for effective treatment of flushed manure should be able to operate at ambient temperatures, tolerate much shorter HRTs, and handle small amounts of fibrous solids.
Fixed-film anaerobic digesters use an internal support media to provide large surface areas for bacterial attachment. Thus, a greater concentration of bacteria is available to mediate the degradation of organic matter. This allows bacterial residence time to be maintained independently of the HRT of the liquid phase. Using much higher concentrations of attached bacteria allows fixed-film digesters to operate at much shorter HRTs and at much lower temperatures while achieving similar treatment efficiencies as conventional plug-flow and complete-mix systems. Currently, designs for high-rate anaerobic processing systems that use fixed-film digesters are available. However, none of the existing fixed-film designs are suitable for wastewaters with significant levels of suspended solids, such as those found in flushed manure. Suspended solids loading for existing fixed-film reactors are limited to less that 10% of the influent chemical oxygen demand (COD).
Livestock manure often includes materials used for bedding, such as hay, sawdust or sand. Often, such materials are poorly degraded or non-biodegradable. Where manure is in liquid form, the liquid is often conveyed into a “lagoon” after separation using solid-liquid separation equipment, with the resultant solids spread on land. Manure presents a complex substrate for anaerobic digestion because the volatile solids content is comprised of readily digestible soluble materials; fine particles that have a high surface-to-volume ratio and are readily accessible to bacterial enzymes; and larger fibrous particles that are relatively inaccessible to microbial attack. These larger fibrous particulates can contribute to clogging of packing material or media. The larger fibrous particulates can also hinder the attachment of bacteria to the media. Ultimately, these situations can lead to short-circuiting of the anaerobic system, which reduces the effectiveness of the biological treatment system. In addition, scum formation is a problem as well as blockage of pipes and other ancillary equipment caused by floating and suspended solids.
For example, certain anaerobic processing systems, such as those disclosed in U.S. Pat. No. 4,183,809, provide for anaerobic microorganisms suspended in a liquid medium to which wastewater is fed. Such processing systems, also known as upflow anaerobic sludge blanket (UASB) reactors, rely on the tendency of anaerobic microorganisms to form flocs or granules (sludge), which are retained within the system by an efficient gas/solids/liquid separation device. Unfortunately, with this system, the microorganisms may be washed out along with the effluent when high levels of particulates are contained in the wastewater. Because of this and the fact that it is difficult to obtain granular sludge with flushed manure, these systems have not been implemented for managing livestock manure.
In other anaerobic processing systems known as fluidized or expanded-bed reactors, the microorganisms are retained within the processing system by attachment to small inert particles (or “packing material”). Suitable particles include sand, anthracite, granular activated carbon, PVC particles, or diatomaceous earth. For example, U.S. Pat. Nos. 3,846,289, 3,956,129, 4,009,099, 4,284,508, and 5,232,585 disclose methods and apparatuses for denitrifying wastewater using solid particulate carriers where particle size generally ranges from about 0.2 to 3 millimeters. Such systems, however, suffer from washout of media and/or reduced media separation efficiency when wastes with suspended solids are treated.
Additional known anaerobic processing systems immobilize the microorganisms on a matrix within the reactor, called fixed-bed reactors. As disclosed in U.S. Pat. Nos. 4,366,059, 4,530,762, 4,561,168, and 4,599,168, the matrix is composed of an inert packing material, or “media,” to provide a surface for microorganism attachment and biofilm development. Unlike the fluidized system and the expanded-bed systems described above, the media includes sheet, ring, or spherical material configured in either a random-pack or an oriented arrangement.
Random-pack (or “loose-fill”) media include such materials as gravel, wood chips, or special plastic pieces designed with a high “surface to air volume” ratio and are packed in loose-fill configuration in fixed-bed reactors. For example, U.S. Pat. Nos. 4,366,059, 4,780,198, and 5,419,833 disclose random-pack media of plastic rings or cylinders. As with the expanded-bed reactors, the random-pack media have poor hydraulic properties when applied to flushed manure. In particular, random-pack materials tend to clog quickly due to the recalcitrant suspended solids often found in flushed manure (e.g., animal hairs, grain husks, fibrous particles and inorganic precipitates), which causes the wastewater to cease to flow evenly through the media, reducing the effective treatment capacity.
In contrast, oriented (or “ordered”) media provide improved hydraulic properties with certain waste products. Known oriented media include materials such as those disclosed in U.S. Pat. Nos. 4,530,762, 4,599,168, 5,228,995, and 6,126,816 for perforated PVC sheets, which are configured in modular blocks with cross-flowing channels located within the blocks. Although current oriented media provide improved hydraulic properties compared to those systems described above, oriented media still tend to suffer from the problem of clogging with respect to aqueous wastes that have significant levels of suspended solids, such as flushed manure. These modular block media promote the settling of suspended particles by decreasing the vertical distance the particles must travel before striking a surface. This promotes accumulation of solids on the media, which impairs biofilm interaction with the wastewater.
Anaerobic digesters with flexible film media and mechanical agitation for flushing/mixing within the system to address the issue of clogging in media have been described (U.S. Pat. Nos. 5,096,579; 6,254,775). These systems are designed for manure with 8-10% TS and are not applicable to flushed livestock waste. Also, these systems operate at a HRT of 28-30 days. Thus, no anaerobic fixed-film bioreactor system currently exists that can effectively treat flushed livestock manure.